This invention relates to a method of testing a vehicle acceleration sensor, included in an anti-lock brake control system for motor vehicles.
Hydraulic brake systems for motor vehicles are usually equipped with some form of anti-lock brake control system, or anti-skid system, in order to prevent wheel lockup during braking. Skidding will occur if the brakes lock the vehicle wheels. Japanese Patent Laid-Open Publication No. 60-61354 represents an example of anti-lock brake control systems so far suggested.
Generally, in hydraulic brake systems, brake fluid is sent from a master cylinder to wheel cylinders at respective vehicle wheels upon application of a brake pedal. The wheel cylinders act to slow or stop the revolving wheels, which in turn slow or stop the vehicle. The anti-lock brake control system acts to stop the supply of the brake fluid to the wheel cylinders, or to release the fluid pressure in the wheel cylinders, as required by the deceleration of the wheels or by the decrease of the wheel speed in relation to vehicle velocity. The wheel speed will build up again and come close to the vehicle velocity as a result of reaction from the road after the fluid pressure in the wheel cylinders is released. Thereupon the control system will restart the delivery of the brake fluid to the wheel cylinders. The same cycle of brake pressure control is repeated thereafter for the most efficient braking.
The above cyclic application and release of the brake pressure as required by the wheel speed have a problem, however. The vehicle is efficiently braked in this manner only when traveling on a road that has a relatively high coefficient of friction and from which, in consequence, the vehicle wheels receive relatively high reaction. When the vehicle is traveling on a road having a lower coefficient of friction, the wheels tend to lock upon application of slight braking effort. The wheel speed will not readily build up because of insufficient reaction from the road after the fluid pressure in the wheel cylinders is reduced by the brake control system. The vehicle must then remain in the state of reduced brake pressure for an extended period of time. Furthermore, as the control system permits the redelivery of the brake fluid to the wheel cylinders to some extent after the wheel speed has built up again, the resulting braking forces will be more than that by the road. The wheels will then skid.
The above discussed problem becomes more prominent when the drive wheels of the vehicle are subject to engine braking. Thus, more particularly, four wheel drive vehicles have been affected by this drawback of the anti-lock brake control system.
Recently, therefore, it has been suggested to incorporate a vehicle acceleration sensor into the anti-lock brake control system. When the vehicle is braked on a road with a low coefficient of friction, the deceleration of the vehicle sensed by the acceleration sensor will be less than a predetermined limit. Then the control system either delays the repressurization of the wheel cylinders or incrementally pressurizes the wheel cylinders in order to defeat the noted shortcoming of the conventional control system.
One known form of the vehicle acceleration sensor is a mercury switch having a mass of mercury contained in a tilting tubular container. The deceleration of the vehicle causes the mercury to move within the container and bridge the switch contacts therein. Conventionally, this and other types of vehicle acceleration sensors have been tested for proper functioning before they are mounted in place on motor vehicles. Such tests are unsatisfactory because there is no guarantee that the tested sensors will function properly after they are mounted on motor vehicles as parts of the anti-lock brake control system.